This invention relates generally to the field of optical fiber design and, more specifically, to optical fiber with a cross-sectional profile for use with optical fiber gain or loss media having improved gain or loss characteristics.
An optical fiber gain medium is a device that increases the amplitude of an input optical signal. If the optical signal at the input to such an amplifier is monochromatic, the output will also be monochromatic, with the same frequency. A conventional fiber amplifier comprises a gain medium, such as a glass fiber core doped with an active material, into which is coupled to an input signal. Excitation occurs from the absorption of optical pumping energy by the core. The optical pumping energy is within the absorption band of the active material in the core, and when the optical signal propagates through the core, the absorbed pump energy causes amplification of the signal transmitted through the fiber core by stimulated emission. Optical amplifiers are typically used in a variety of applications including but not limited to amplification of optical signals such as those that have traveled through a long length of optical fiber in optical communication systems.
Many different types of optical fiber have been developed over the years to improve the characteristics of fiber gain media. Different characteristics of the fiber can significantly affect the various characteristics of concern. For example, doping with different active materials produces different wavelength profiles for both absorption of pump energy and gain. In a silica-based fiber, doping with erbium produces an absorption spectrum, for example, in the ranges of 980 nm and 1480 nm, and an emission spectrum, for example, in the range of 1550 nm. However, co-doping with erbium and ytterbium produces a gain medium that, due to the charge transfer between the ytterbium and the erbium, produces an absorption spectrum that includes light in the vicinity of 1060 nm, while still maintaining an emission spectrum in the vicinity of 1550 nm.
Since signal propagation is in the core of a fiber, doping of the fiber with an active material is typically in the core. Pump light is therefore often coupled into the core as well. In some multiple-clad fibers, however, signal propagation is in the doped fiber core, while pump light is injected into an inner pump cladding of the fiber that surrounds the core. It is still the core that is the portion of the fiber doped with active material. However, the pump light traveling through the inner cladding tends to intersect the core, getting absorbed by the active material. This produces the desired gain in the core.
Recently, fiber has been described that has a core doped with active material, but that also has a cylindrical ring separated from and encompassing the core that is also doped with an active material. An example of such an arrangement is shown in U.S. Pat. No. 5,970,198, in which a fiber has a core doped with a first combination of materials, and a ring region spaced radially from the core and doped with a different combination of materials. As such, the two doped regions produce different gain responses when pumped. The core of the fiber is pumped with pump energy at two different wavelengths, a first, shorter wavelength (i.e., 0.98 xcexcm) and a second, longer wavelength (i.e., 1.48 xcexcm). Because of their different wavelengths, each of the pump signals has a different mode field diameter. That is, each pump signal has a different distribution of power density over the cross-sectional area of the fiber. The portion of the light that extends beyond the core boundary is generally referred to as the xe2x80x9cevanescentxe2x80x9d portion. The longer wavelength pump signal has a much larger evanescent portion, and therefore a much greater interaction with the dopant in the ring region. Therefore, adjusting the relative power of the two pump signals allows the overall gain profile of the gain medium to be adjusted.
In accordance with the present invention, an optical fiber gain medium is provided that has a core region within which an optical signal propagates, and a cladding region surrounding the core region. The cladding region has a lower index of refraction than the core region, and may be appropriate for receiving pump energy that pumps an active material in the gain medium so as to provide signal gain to the optical signal in the core. The fiber gain medium also has a plurality of doped regions within the fiber that influence the gain profile of the gain medium.
In a first embodiment of the invention, first and second doped regions are located within the cladding region of the fiber. Each of the doped regions has a different radial distance from the core, and each has a different dopant composition. Each of the dopant compositions results in an absorption loss to the optical signal in the core, as an evanescent portion of the signal overlaps and interacts with each of the doped regions. In this embodiment, the absorption loss attributable to interaction with the first doped region affects a different wavelength range than that attributable to the second doped region.
Different mechanisms may be used to effect the degree to which different wavelengths in the signal are absorbed by the doped region. For example, the different radial positions of the rings may be used to provide different degrees of signal absorption at different wavelengths, since the different wavelengths will have different relative degrees of evanescent spreading into the cladding region. The materials used to dope the doped regions may themselves be selected to create a desired absorption profile, either in and of themselves, or in combination with the radial positioning of the doped regions. The density of the dopant distribution in each doped region may also be selected to control the relative effect of each. Moreover, the radial width of the doped regions can also be used to control the relative absorption in different wavelengths of the optical signal, and may be combined with other methods of doing so.
In another embodiment of the invention, the fiber has a core region and a cladding region, and multiple doped regions include a doped region in the core and a doped region in the cladding. Each of the doped regions includes an active material that, when pumped with pump energy at an appropriate pump wavelength, provides gain to the optical signal in the core within a desired wavelength range. Also provided in this embodiment are a first pump source coupling a first pump signal into the core, and a second pump source coupling a second pump signal into the cladding region. Independent control of the first pump source and the second pump source allow the relative gain contributions of the two doped regions to be actively controlled. The radial distance of the doped cladding region from the core may be selected to provide desired relative contributions from the two doped regions. The width of the doped cladding region may also be selected to affect the relative contributions of the doped core region and the doped cladding region to the signal gain. Similarly, the relative density of the dopant in the cladding region and the dopant in the core may likewise be used to control the relative gain contributions from the two regions. Also, the particular dopant material for either or both of the doped regions may be selected so as to provide significantly more gain to one wavelength range within the signal gain band than to other wavelengths within the gain band. Any of these different methods may be mixed and matched to provide a desired gain profile for the gain medium.